R-curve behavior

2 Mechanical Properties of Ceramic Composites R-Curve Behavior 

Rgure 10.3 TEM micrographs of aluminas (A) containing (a) glass (G) and (b) crystalline grain boundaries (C).  

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Specimen size effects have been attributed to rising R-curve behavior for most graphites studied. 

Rising R-curve behavior has been attributed to grain bridging traetions and microcrack dilatation in the eraek wake region. 

Crack growth resistance (R-curve) behavior in transverse fracture... 

Fig. 2.18 The R-curve behavior of a woven Nicalon/SiC composite at ambient and elevated temperatures.82...

C. W. Li and J. Yamanis, Super-Tough Silicon Nitride with R-Curve Behavior, Ceram. Eng. Sci. Proc., 10[7-8], 632-645 (1989). 

Nalla, R.K., Kruzic, J.J., Kinney, J.H., and Ritchie, R.O. (2005) Mechanistic aspects of fracture and R-curve behavior in human cortical bone. Biomaterials, 26, 217-231. 

Figure 11.19 (a) Functional dependence of fracture toughness on flaw size for a ceramic exhibiting R curve behavior (top curve) and one that does not (lower curve), (h) Effect of R curve behavior on strength degradation as flaw size increases. Ceramics exhibiting R curve behavior are more flaw-tolerant than those that do not. 

There are four important implications for ceramics that exhibit R curve behavior ... 

On the down side, there is now an increasing body of evidence that seems to indicate that ceramics that exhibit R curve behavior are more susceptible to fatigue than ceramics that do not exhibit R curve behavior. This is discussed in greater detail in Chap. 12. 

There is some recent evidence to suggest that R curve behavior enhances the thermal shock resistance of some ceramics. The evidence at this point is not conclusive, however, and more work is needed in this area. 

Cmin/cmax which is clcarly independent of absent R curve effects. Thus toughening of a solid per se will often not result in an increase in its Weibull modulus. However, it can be easily shown that if a solid exhibits R curve behavior, then an increase in m should, in principle, follow (see Prob. 11.12). 

In this equation, P is the indentation load, tension surface at failure, and r] is fitting coefficient, =0.62. Since P oc c (c crack length), an increase in fracture toughness with indentation load signifies an increased resistance to crack propagation with increasing crack length so called R-curve behavior. 

H. Kroninger and A. Reynolds, R-Curve Behavior of Friction Stir Welds in Aluminum-Lithium Alloy 2195, Fatigue Fract. Eng. Mater. Struct, Vol 25, 2002, p 283-290... 

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